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34. the K-Replacement Origin of the Megacrystal Lower Caribou Creek Granodiorite and the Goat Canyon-Halifax Creeks Quartz Monzo

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34. the K-Replacement Origin of the Megacrystal Lower Caribou Creek Granodiorite and the Goat Canyon-Halifax Creeks Quartz Monzo 1 ISSN 1526-5757 34. The K-replacement origin of the megacrystal Lower Caribou Creek granodiorite and the Goat Canyon-Halifax Creeks quartz monzonite --- modifications of a former tonalite and diorite stock, British Columbia, Canada Lorence G. Collins email: [email protected] February 8, 1999 Introduction About 25 km south of Nakusp and 1 km east of Burton, British Columbia, Canada, is a stock whose western half is the Lower Caribou Creek megacrystal granodiorite and whose eastern half is the Goat Canyon-Halifax Creeks fine- grained quartz monzonite (Fig. 1); Hyndman, 1968). Biotite-hornblende tonalite and diorite are additional facies that occur along the border of the stock. Zoned plagioclase in all these rocks indicates a shallow intrusion and relatively rapid crystallization. This stock was called to my attention by Donald Hyndman because the K-feldspar megacrysts in the granodiorite appeared to be orthoclase whereas the smaller K-feldspar crystals in the fine-grained quartz monzonite were microcline. Subsequently, he loaned me 19 thin sections from his Ph.D thesis study (Hyndman, 1968), and these sections provided a broad representation of the textural and compositional variations in both rock types. Because he indicated that the two granitic facies were gradational to each other, this stock offered an opportunity to study the field and textural relationships across the transitions between the rocks having the two kinds of K-feldspar. Following his loan of thin sections, I visited the area and obtained 44 additional samples (and thin sections) across transitions between the two rock types. Examinations of field relationships and both his and my thin sections provide the basis for the conclusions presented in this article. 2 Fig. 1. Geologic map of the stock consisting of the Lower Caribou Creek megacrystal granodiorite (rectangular block pattern; green), the Goat Canyon- Halifax Creeks quartz monzonite (v-pattern; blue), and biotite-hornblende tonalite and diorite border facies (random dash pattern; red); modified after Hyndman (1968 but omitting many other mapped rock types. The highway from Nakusp to Burton and forest roads extending into the map area is not shown. Hyndman's HQ- samples and areas A, B, and C, indicated by solid dots, are discussed in the text. Field relationships Before going to the stock near Nakusp, I had imagined from the distribution of rock types on the geologic map (Fig. 1) that the intrusion pattern would be that of a typical zoned pluton created by magmatic differentiation processes. That is, 3 the rim would be a more-mafic rock, the diorite, and progressively toward the center, the rocks would gradually become more felsic with the quartz monzonite first and finally the megacrystal granodiorite. This seemed logical although the map pattern showed an asymmetric distribution with the diorite primarily along the northeast, east, and southeast sides and the granodiorite on the west side (Fig. 1). I also pictured the contact between the fine-grained monzonite and the megacrystal granodiorite as being a gradual smooth transition, beginning approximately where Hyndman drew the separation line (Fig. 1). On the basis of these assumptions, I was surprised to discover when I arrived that the mapped distribution of the various rocks only existed in a broad sense. The transition westward from the fine- grained quartz monzonite to the megacrystal granodiorite was not a single gradual smooth transition but consisted of several gradual transitions, back and forth, between the two rock types before finally becoming mostly megacrystal granodiorite farther west. Moreover, in this transition interval, in both the megacrystal granodiorite and the quartz monzonite were small unmapped units of diorite. Such details were not essential to Hyndman's study of the larger area that he mapped and were impossible to delineate at the small scale of his map. Nevertheless, the discovery of this distribution of rock types meant that the diorite did not just form on the rims of the pluton but was also present farther into the middle and that a magmatic differentiation model for the origin of the pluton with gradual compositional changes to a more-felsic core was not entirely consistent. This conclusion was verified when I examined the contact relationships among diorite, fine-grained quartz monzonite, and megacrystal granodiorite in outcrops in area A (Fig. 1). In some places in the transition zone, the megacrystal granodiorite grades into fine-grained quartz monzonite within a few centimeters and then appears to penetrate into the diorite as an intrusive body (Fig. 2, Fig. 3, and Fig. 4). Some of the diorite contains angular disoriented blocks of diorite that vary from light green to dark green to black. The disoriented blocks indicate that the diorite has been strongly sheared, and the blocks have experienced different degrees of alteration to chlorite and epidote. Within the diorite mass, however, locally there are tiny isolated grains or small aggregate patches of pink K-feldspar that have no apparent physical connection to the main granodiorite mass in the sense of being transported there via magma. 4 Fig. 2. Chloritized and epidotized diorite, penetrated by fine-grained granitic rock which locally contains megacrysts of K-feldspar. Canadian penny provides a scale. Area A, Fig. 1. 5 Fig. 3. Megacrystal granodiorite (right side) has an irregular contact with a cataclastically deformed diorite, altered by chlorite and epidote (left side). Relatively unaltered and undeformed diorite at bottom of photo has a sharp contact with the granodiorite. Area A, Fig. 1. 6 Fig. 4. Close-up of contact of fine-grained quartz monzonite and megacrystal granodiorite with the deformed diorite. The close-up area is in the lower left near the black, less-sheared diorite (bottom of photo in Fig. 3). Canadian penny provides a scale. Area A, Fig. 1. Not all of the diorite is sheared, however. In the same outcrop, a sharp contact of the megacrystal granodiorite against relatively unaltered and unsheared black diorite can also be seen. See the lower part of Fig. 3. In other parts of the transition zone, the fine-grained quartz monzonite grades into megacrystal granodiorite gradually across several tens of meters. In this gradual transition, the randomly oriented K-feldspar megacrysts that first appear are only 0.5 to 1.0 cm long. Farther into the megacrystal granodiorite, the crystals may increase in size to 2-3 cm long (Fig. 5). 7 Fig. 5. Massive megacrystal granodiorite, showing random orientation of light pink K-feldspar megacrysts in a matrix of white plagioclase, gray quartz, and black biotite and hornblende. Canadian penny provides a scale. Area A, Fig. 1. In many places at area B on Fig. 1, in bulldozed road outcrops, the repeated gradual transitions, back and forth, between fine-grained quartz monzonite and megacrystal granodiorite are also found. Here also, diorite or tonalite is found adjacent to the fine-grained quartz monzonite. Contacts are usually relatively sharp, but in a few places are gradational. At area C on Fig. 1, similar transitions from fine-grained quartz monzonite to megacrystal granodiorite and/or to remnants of diorite and tonalite are also exposed. Here, also, contacts of the granitic rocks against the more mafic rocks tend to be relatively sharp. In a few places megacrysts of K-feldspar occur in the diorite and tonalite within 10 centimeters (or less) from the contact with the megacrystal granodiorite. 8 Thin section analyses and discussion of the Lower Caribou Creek megacrystal granodiorite Thin sections of the sheared diorite (Fig. 2, Fig. 3, and Fig. 4), show that portions of this rock are strongly granulated to become a cataclasite in which many of the grains are altered to chlorite and epidote. In the fine-grained quartz monzonite adjacent to the altered diorite (Fig. 2, Fig. 3, and Fig. 4), small microcline crystals (0.5 -3 mm wide) contain angular remnants of plagioclase which are in optical parallel orientation (Fig. 6). Fig. 6. Microcline (black, dark gray) enclosing irregular islands of albite-twinned plagioclase (light gray) in parallel optical orientation. Area A, Fig. 1. Where the K-feldspar megacrysts (0.5 to 1 cm) first appear (areas A and B, Fig. 1), the interior remnant islands of plagioclase no longer occur (as in Fig. 6), but tiny islands may remain along the K-feldspar borders in optical parallel continuity with the adjacent larger plagioclase crystal. Moreover, the K-feldspar may extend in veins into the adjacent plagioclase (Fig. 7, Fig. 8, and Fig. 9). 9 Fig. 7. Microcline (black), penetrating plagioclase (light gray) along veins and enclosing tiny islands of plagioclase (cream white; left of center) which are in optical parallel continuity with the adjacent plagioclase. Area A, Fig. 1. 10 Fig. 8. Microcline (black), penetrating plagioclase (light gray) along veins and enclosing tiny islands of plagioclase (cream white) which are in optical parallel continuity with the adjacent plagioclase. Area A, Fig. 1. 11 Fig. 9. Microcline (black), penetrating plagioclase (light cream) along veins and along albite-twin lamellae, leaving some lamellae projecting into the microcline. A portion of the plagioclase that is enclosed in the microcline is myrmekitic with tiny quartz ovules. Myrmekite also occurs in upper right. Area B, Fig. 1. The same textural relationships are found in the fine-grained quartz monzonite and megacrystal granodiorite at area C (Fig. 1) as in areas A and B, except that in area C in a few places megacrysts of microcline also occur bordering diorite or tonalite for the first 10 cm beyond the contact. Here, also, island remnants of optically parallel plagioclase occur in the microcline, and the microcline penetrates the adjacent plagioclase along veins (Fig. 10). Farther into the diorite or tonalite where K-feldspar megacrysts are absent, microcline is interstitial, bordered by myrmekite, and also penetrates broken plagioclase grains along veins or occurs in interiors of deformed plagioclase grains in irregular random islands.
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